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The partial trace can be viewed as a quantum operation. Consider a quantum mechanical system whose state space is the tensor product H A ⊗ H B {\displaystyle H_{A}\otimes H_{B}} of Hilbert spaces. A mixed state is described by a density matrix ρ , that is a non-negative trace-class operator of trace 1 on the tensor product H A ⊗ H B ...
the reduced state of ρ on system A, ρ A, is obtained by taking the partial trace of ρ with respect to the B system: =. The partial trace operation is a CPTP map, therefore a quantum channel in the Schrödinger picture. [5] In the Heisenberg picture, the dual map of this channel is
The quantum mechanical counterpart of classical probability distributions are modeled with density matrices. Consider a quantum system that can be divided into two parts, A and B, such that independent measurements can be made on either part. The state space of the entire quantum system is then the tensor product of the spaces for the two parts.
Quantum state tomography is a process by which, given a set of data representing the results of quantum measurements, a quantum state consistent with those measurement results is computed. [50] It is named by analogy with tomography , the reconstruction of three-dimensional images from slices taken through them, as in a CT scan .
Moreover, a mixed quantum state on a given quantum system described by a Hilbert space can be always represented as the partial trace of a pure quantum state (called a purification) on a larger bipartite system for a sufficiently large Hilbert space .
A quantum depolarizing channel is a model for quantum noise in quantum systems. The d {\displaystyle d} -dimensional depolarizing channel can be viewed as a completely positive trace-preserving map Δ λ {\displaystyle \Delta _{\lambda }} , depending on one parameter λ {\displaystyle \lambda } , which maps a state ρ {\displaystyle \rho } onto ...
Generation of the 3-qubit GHZ state using quantum logic gates.. In physics, in the area of quantum information theory, a Greenberger–Horne–Zeilinger (GHZ) state is an entangled quantum state that involves at least three subsystems (particle states, qubits, or qudits).
The purity of a quantum state is conserved under unitary transformations acting on the density matrix in the form †, where U is a unitary matrix. Specifically, it is conserved under the time evolution operator U ( t , t 0 ) = e − i ℏ H ( t − t 0 ) {\displaystyle U(t,t_{0})=e^{{\frac {-i}{\hbar }}H(t-t_{0})}\,} , where H is the ...